1. Technical Field
The present invention relates to structural skin. In particular, the present invention relates to methods and apparatuses for stiffening structural skin.
2. Description of Related Art
Structural skin is often used in manufacturing large parts, such as aircraft wing torque boxes, fuselages or control surface structures. This type of structure utilizes thin skins that would not be stable under bending and torsion loads that produce significant shear or compression in the walls. This type of construction is typical of most aerospace structures including wings, fuselages, control surfaces, tail booms, etc. Structural skins can be made thinner, and are, therefore, more weight efficient, when internal stiffening elements are used. A rib, for example, is a structural stiffening element that is disposed perpendicular to the longitudinal axis of a box beam, i.e., the rib lies in a cross-sectional plane of the beam structure. Ribs serve a variety of purposes in thin-skinned structure, including: (1) to provide support for the skin/skin stringer or spar panels against catastrophic buckling; (2) to maintain shape and contour of the skin; (3) to provide stiffness at major load introduction points; (4) to distribute concentrated loads into surrounding thinner structure; (5) to provide a shear redistribution path in the case of failure of any structural elements; and (6) to distribute pressure into the skin. These ribs are typically located at major load introduction points. In most instances, the entire rib is used to react loads; however, in some instances only certain regions of the rib is used to react loads. In addition, some ribs do not have any load introduction points, but react internal pressure loads.
Assembly of these structural box beams can be very complex, often with very tight tolerances required. As the number of parts is reduced, the manufacturing tolerances become even more critical, because there are fewer joints where variances can be accommodated. The installation of fasteners into these box beams presents additional difficulties, including limited access to small interior spaces and complicated sealing requirements.
One-piece closed cell structures can economically be produced with a variety of methods, including filament winding, automated tape placement, resin transfer molding, and others. However, these assembly-tolerance issues often preclude the use of one-piece closed-cell torque box structures with secondarily attached internal ribs, i.e., slipped-in ribs. Because of the reduction in part count and assembly labor associated with consolidating the torque box skins into a single part, a substantial cost savings could be realized if the assembly tolerance issues could be overcome. Several composite fabrication technologies are available to economically produce such a joint-free torque-box structure, including filament winding, automated tape placement, resin transfer molding, and others. Practical application of one-piece, jointless torque box structures has been limited because of the difficulty of installing the internal stiffening ribs. A rib installation design that allowed for large assembly tolerances and the resulting gaps between the rib and the torque box skins would enable more widespread application of these cost-saving technologies.